Method for processing OFDM signals simultaneously received via a multiple antenna system

ABSTRACT

The aim of the invention is to process OFDM signals which are simultaneously received via a multiple antenna system having a plurality of separate receive channels. To this end, the channel correction values or the confidence values are determined in the receive channels for each carrier of the OFDM signal while using known techniques. In addition, the I/Q values of each individual carrier of the OFDM signal which are obtained in the OFDM demodulator are weighted with these channel correction values or confidence values. Afterwards, the I/Q values weighted in such a manner are totaled.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/EP99/07102, which has an Internationalfiling date of Sep. 23, 1999, and which designated the United States ofAmerica.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for processing OFDM-signalsreceived simultaneously by a multi-antenna system with a plurality ofseparate receiving channels.

2. Description of the Background Art

In modern digital technology, so-called OFDM-systems(Orthogonal-Frequency-Division Multiplex) or COFDM-systems (coded OFDM)are used for data transmission (sound, video or other data). Inaccordance with this principle, prior to transmission the digital datastream is split via a transmitter network into a plurality ofsub-signals, each of which is transmitted separately on an individualcarrier. In the so-called DVB-T-system (Digital-Video Broadcasting,terrestrial), which also serves for the transmission of data of ageneral type, 1705 or 6817 individual carriers are used for example. Inthe receiver these items of subsidiary information are recombined toform a complete item of information of the transmitter-end digital datastream.

These OFDM-systems are standardized in terms of the transmitting-endconditioning and receiving-end recovery of the data (for example in theDAB-standard ETS 300401 for DAB and in the standard ETS 300744 forDVB-T). It is a common feature of these OFDM-systems that at thereceiving end the high-frequency signal received by an antenna isdemodulated in an OFDM-demodulator, preferably after conversion into anintermediate frequency, and in this way the associated I/Q-values areacquired for each individual carrier. In a so-calledpilot-tone-corrected OFDM-system, as used in DVB-T, a channel correctionvalue is determined simultaneously from the co-transmitted pilot tones.For each individual carrier, each I/Q-value is complexly multiplied bythe relevant channel correction value. This ensures that all thecarriers have constant amplitudes, possible breaks in amplitude ofindividual carriers of the overall reception band, caused for example bymultipath reception disturbances, being appropriately compensated andcorrected.

In such systems, in addition to the individual data, it is also commonpractice to transmit so-called confidence values and thus to influencethe further processing of the acquired digital values in so-calledsoft-decision processes. These two known possibilities of correcting theI/Q-values via the channel correction or the obtained digital valuesthrough the confidence values are state of the art in receivertechnology.

To improve the signal/noise ratio, in particular for the mobilereception of such OFDM-signals, it is known to provide a multi-antennasystem with two or more antennae and correspondingly assigned, separatereceiving channels, and to combine the analogue received signals in thereceiver in the HF- or IF-plane of this plurality of receiving channels.The analogue signals of the individual receiving channels are added,having been weighted in a frequency-dependent manner, for example, as afunction of the received power. Here, however, not only the usefulsignals but also the noise components are combined, which in principlecan even result in an impairment of the signal/noise ratio compared tothe most favourable receiving channel for the relevant sub-band. Theseanalogue combining processes also require a very high outlay and followthe relevant channel properties only relatively slowly. In the case offrequency-selective addition, they have only relatively flat selectioncurves, i.e. sharp breaks in the receiving frequency range cannot becorrected.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention is to indicate aprocess for combining OFDM-signals received simultaneously by amulti-antenna system which avoids these disadvantages and leads to adistinct improvement in reception.

In accordance with the invention, in each individual receiving channelof the multi-antenna system, the values for channel correction orconfidence anyhow acquired therein according to the relevant standardare used for a corresponding weighting of the demodulated I/Q-values. Inthe DAB-system, in which the confidence values are determined in a knownmanner, these can be used in accordance with the invention to add therelevant I/Q-values in an appropriately weighted manner and thus, fromthe relevant receiving branches having a good signal/noise ratio for thereceived signal, to obtain a corresponding mean value of the individualreceived signals of the multi-antenna system, which is particularlyadvantageous for the mobile reception of DAB-signals where, due to theproperties of the transmission channel, a more difficult receptionsituation exists than in the case of stationary reception. In this wayfading disturbances can be corrected.

It is particularly advantageous to perform this correction as a functionof the channel correction values as provided in the DVB-T-system. Hereagain, mobile reception with a good signal/noise ratio is possible, thisweighted evaluation of the received signals in the individual receivingchannels facilitating a particularly simple analysis algorithm.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not limitativeof the present invention, and wherein:

FIG. 1 is a circuit diagram of the present invention according to apreferred embodiment;

FIG. 2 is a graph illustrating an example of weighting and adding fortwo antenna; and

FIG. 3 is a circuit diagram of the present invention according to analternate embodiment.

DETAILED DESCRIPTION

FIG. 1 is the fundamental circuit diagram of a receiving arrangement forprocessing pilot-tone-supported OFDM-signals in which the signals of theindividual carriers of the multi-antenna system are digitally combinedupstream of the decision device. The received multicarrier OFDM-signalsare received via a plurality of antennae A1 to An and can optionally beconverted into a suitable intermediate frequency via individualreceivers E1, E2 to En. All the receivers E1 to En are set at the samereceiving frequency and for simplicity the down-conversion into theintermediate frequency optionally can be performed using a commonoscillator. Then, in each of the n receiving channels, the demodulationof the OFDM-signals is in each case performed in separate demodulatorsD1 to Dn and at the same time the associated channel correction valuesare also acquired, these being a gauge of the level of the individualcarriers of the multicarrier system and thus also a gauge of theprobability that the symbol transmitted with this carrier is correct.

The I/Q-values available for each individual carrier at the output ofthe demodulators are fed to a time synchronisation device S, in whichpossible time offsets of the total number n of I/Q-signals are correctedby corresponding delay devices so that the I/Q-values of correspondingcarriers occur simultaneously at the output of this time synchronisationdevice S, which values are then fed to a processing device R andprocessed therein as will be described in the following. The timesynchronisation can be performed using synchronisation flags known inassociation with OFDM demodulators.

Before the I/Q-values, thus conditioned in known manner, are reduced toindividual bits in the decision (demapping) device M, in the processingdevice R they are complexly multiplied by a value k proportional to thereciprocal value of the relevant channel correction, and thus areweighted. This weighting is firstly performed individually, for eachI/Q-value for all n receiving channels. The I/Q-values are thus weightedparticularly high if they are changed as little as possible by thechannel correction. Then all the mutually assigned I/Q-values are addedand divided by the sum of all the weights. FIG. 2 illustrates this typeof weighting and addition for two antennae A1 and A2. Of the total of1705 or 6817 individual carriers of the system, at the frequency f1 thereceived carrier is received only with a reduced amplitude via theantenna A1 due to fading. This is expressed by the reciprocal value k1of the channel correction obtained for this receiving channel. Thecarrier at the frequency f1 is thus weighted with a relatively lowweight, for example only with the channel correction value 2, while thecarriers in the range below and above the frequency f1, which arereceived at the full level, are weighted very high, for example with theweigh 10. In the case of the antenna A2 this low weighted receivingrange lies at a different location at the frequency f2.

When the I/Q-values, differently weighted in this way with for example 2and 10 in the f1 range, are now added and finally divided by the totalnumber of all the weights (in the example 12), a mean value is obtainedwhich has a constant good reception value over the entire frequencyrange. The averaged I/Q-values thus obtained in the processing device Rare then fed to the decision device M and further analyzed therein inknown manner. Optionally, the confidence of the information can also becalculated therein. Then the data are further processed in aconventional Viterbi-decoder V with soft decision.

FIG. 3 illustrates an exemplary embodiment of a receiving arrangementfor processing OFDM-signals in a multi-antenna system by digitalcombination downstream of the decision device M. In many cases theI/Q-values are available for further processing not upstream of thedecision device but only downstream of the decision device M, which inthis case for example is integrated in the demodulator D1 for eachindividual receiving channel. Thus the data words reduced to individualbits are already available at the output of the demodulator and indeedtogether with the confidence values which have likewise been calculatedin the decision devices M1 to Mn and which, following the timesynchronisation in the time synchronisation device S, are weighted andfurther processed in the processing device R as follows.

Each individual data word of the n receiving channels is reduced to theoriginal I/Q-values using a suitable algorithm. The thus obtained,corresponding I/Q-values are then complexly multiplied by the value ofthe relevant confidence information, whereupon all the thus weightedI/Q-values are added again as described in association with FIG. 2 andthen divided by the number of all the weights. When the I/Q-values havebeen reduced to the data bits, the thus determined mean value of all theI/Q-values is then fed again to the Viterbi-decoder with soft decision Vand further processed.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

1. A process for processing OFDM-signals, comprising the steps of:receiving substantially simultaneously the OFDM-signals by amulti-antenna system with two or more corresponding separate receivingchannels; in each separate receiving channel, obtaining the I/Q-valuesof each individual carrier of the OFDM-signals and determining channelcorrection values or confidence values from pilots for each individualcarrier of the OFDM-signals; deriving weighting factors from the channelcorrection or confidence values for each I/Q-value of each individualcarrier and each separate receiving channel, by which weighting factorsthe I/Q-values of each individual carrier of the OFDM-signals obtainedin an OFDM-demodulator are weighted such that I/Q-values of carriersreceived at a low level are weighted low and I/Q-values of carriersreceived at a high level are weighted high; and adding the weightedI/Q-values and dividing the added weighted I/Q-values by the number ofall the weighting factors.
 2. The process according to claim 1, whereinthe demodulated I/Q-values at the output of the OFDM demodulator are fedto a time synchronisation device so that the I/Q-values of correspondingcarriers of the separate receiving channels are in each casesimultaneously available for further processing.
 3. The processaccording to claim 1, wherein the I/Q-values of each individual carrierof the OFDM-signals are weighted as a function of the channel correctionvalues obtained from the pilots, such that low weighting factors areselected for large channel correction values and high weighting factorsare selected for small channel correction values.
 4. The processaccording to claim 1, wherein the weighted I/Q-values are obtained bycomplexly multiplying the I/Q-values with the corresponding confidencevalues.
 5. The process according to claim 1, wherein each of theOFDM-signals received by the multi-antenna system has the same centerfrequency.
 6. A method for receiving and processing OFDM signals, themethod comprising the steps of: receiving substantially simultaneouslythe OFDM signals by a plurality of antennas, each of the plurality ofantennas having a separate receiving channel; demodulating each of thereceived OFDM signals; acquiring channel correction values or confidencevalues for each of the demodulated OFDM signals; providing I/Q valuesfor each of the demodulated OFDM signals; determining a weighting factorfor each of the I/Q values on the basis of the channel correction valuesor confidence values associated with each of the OFDM signals; weightingeach of the I/Q values by the corresponding weighting factor;determining a total weight by adding together the weighting factors ofeach of the weighted I/Q values; adding together each of the weightedI/Q values for each of the demodulated OFDM signals; and dividing theadded weighted I/Q values by the total weight to determine a mean value,the mean value being utilized to maximize the signal to noise ratio ofthe received OFDM signals.
 7. The method according to claim 6, whereineach of the separate receiving channels is set at substantially the samereceiving frequency.